Meta- and para-DIPB are important intermediates in organic synthesis. Thus resorcinol and hydroquinone can be prepared by oxidizing meta- and para-DIPB, respectively, with air and then decomposing the resulting dihydroperoxide with acid. However, although para-DIPB can be separated from a mixture of DIPB isomers by super fractionation, the boiling points of ortho- and meta-DIPB are too close to allow effective separation of meta-DIPB by fractionation. Moreover, ortho-DIPB is not readily oxidized and hence builds up in the production loop, requiring removal as a purge and representing a yield loss. Thus, to be commercially viable, any process for producing DIPB and, in particular meta-DIPB, must minimize the production of the ortho-isomer.
Currently, meta-DIPB is manufactured commercially by alkylating cumene with propylene over a homogeneous AlCl3 catalyst. The high activity of the AlCl3 catalyst produces a mixture of DIPB isomers with near equilibrium ortho content. This is advantageous since at equilibrium in the liquid phase between 50 and 150° C. the ratio of meta:ortho DIPB is greater than 100 providing sufficient purity for efficient downstream conversion to resorcinol. Process operation between 50 and 150° C. also results in DIPB products containing less than 1000 ppm of co-boiling n-propylisopropylbenzene impurities. However, corrosion and the need to neutralize, separate and recycle the AlCl3 catalyst, make it difficult to employ.
DIPB can also be produced by separation from the polyalkylated by-product of the alkylation of benzene with propylene to produce cumene. However, DIPB separated from the polyalkylated fraction of current commercial cumene plants is rich in the kinetically preferred para- and ortho-DIPB isomers, making this route of limited use in the synthesis of meta-DIPB, unless the ortho- and para-content is reduced by, for example, isomerization or transalkylation. Transalkylation and isomerization, however, can introduce contaminant n-propyl-isopropylbenzenes.
Accordingly, there is an outstanding need for a heterogeneous process for producing DIPB rich in the meta-isomer and substantially free of the ortho-isomer and n-propylisopropylbenzenes.
U.S. Pat. No. 4,992,606 discloses a process for preparing short chain (C1-C5) alkylaromatic compounds by alkylation of an aromatic compound, such as benzene and cumene, with a short chain alkylating agent, such as propylene, over the molecular sieve MCM-22. In addition, U.S. Pat. No. 4,962,257 discloses the use of MCM-22 in the disproportionation of toluene to xylenes.
U.S. Pat. No. 5,329,059 discloses a process for the disproportionation of an alkylaromatic compound, wherein the alkyl group has from 1 to about 6 carbon atoms, e.g., cumene, by contacting said compound with catalyst comprising an active form of synthetic porous crystalline MCM-49.
U.S. Pat. No. 4,822,943 discloses a process for the selective production of para-DIPB by reacting cumene and/or benzene with propylene over the molecular sieve ZSM-12.
U.S. Pat. No. 5,198,595 discloses a process for preparing alkylaromatic compounds by alkylation of an aromatic compound with an alkylating agent having two to eighteen carbon atoms, such as propylene, over mordenite which has been subjected to repeated calcination and acid treatment so as to have a silica/alumina molar ratio of at least 40:1.
U.S. Pat. No. 6,049,018 discloses the porous crystalline material MCM-68 and its use in the alkylation of aromatics with short chain (C2-C6) olefins (for example, the alkylation of benzene with ethylene or propylene to produce ethylbenzene or cumene respectively), the transalkylation of aromatics (for example, the transalkylation of polyethylbenzenes or polyisopropylbenzenes with benzene to produce ethylbenzene or cumene respectively), and the disproportionation of alkylaromatics (for example, the disproportionation of toluene to produce xylenes).
U.S. Pat. No. 3,780,123 discloses the catalytic disproportionation of alkylbenzenes, including cumene, by contacting the alkylbenzene and a sulfide compound with hydrogen mordenite containing a sulfided Group VIII metal. According to Table 1 of U.S. Pat. No. 3,780,123, when mordenite is used to disproportionate cumene in the presence of methyldisulfide as the sulfide compound, the process produces a mixture of DIPB isomers in which the meta:ortho isomer ratio is between 58 and 85 and the product contains 4.4-7.2 wt % n-propylbenzene and 4.4-5.2 wt % of unidentified impurities. As a co-boiler with cumene, n-propylbenzene is an undesirable impurity, particularly since, on disproportionation, it yields n-propylisopropylbenzenes which tend to co-boil with meta-DIPB.
It will, of course, be understood that the disproportionation of cumene to produce DIPB and benzene is the inverse of the transalkylation of DIPB with benzene to produce cumene.
According to the invention, it has now been found that the disproportionation of cumene over a wide range of molecular sieve catalysts is unexpectedly more selective towards the production of meta-DIPB and less selective towards the production of ortho-DIPB than the alkylation of cumene with propylene over the same catalysts. Moreover, such molecular sieve catalysts produce relatively low concentrations of undesirable by-products, such as n-propylbenzene and triisopropylbenzenes. In particular, and contrary to the teaching in U.S. Pat. No. 3,780,123, it has been found that mordenite (in the absence of sulfided hydrogenation metal) can disproportionate cumene to produce DIPB in which the meta:ortho ratio is in excess of 500 and the by-product production is less than 1-2% by weight.